PROJECT SUMMARY Our interdisciplinary team proposes to develop and apply a novel method to systematically identify proteins involved in inter-organ communication. While a number of hormone peptides have been identified in mammals, many physiologically important factors involved in energy homeostasis, water-salt balance, pH, and blood pressure remain to be identified. As existing methods of identifying secreted factors from the blood using mass spectrometry (MS) have serious limitations, we will use an engineered promiscuous biotin protein ligase to biotinylate secreted proteins in a specific organ, and then identify biotinylated proteins in distant target organs through affinity enrichment followed by quantitative MS. These experiments will be performed in two organisms, Drosophila and the mouse. Drosophila facilitates the rapid development and optimization of the basic research strategy, generating evolutionary insight into conserved factors in organ communication. The mouse provides a more direct discovery model for biomedically-relevant, inter-organ regulatory communication in humans. In addition to identifying factors involved in organ communication, both in normal and disease conditions, our findings will provide fundamental insight into novel proteins that could be used in the future as therapeutics. Given our goal to broadly interconnect organ systems, the project is relevant to most NIH Institutes.